The technique of coextrusion of at least two thermoplastic materials is well known and it is described, in particular, in POLYMER PLASTICS TECHNOLOGY AND ENGINEERING, Volume 3, pages 49 to 68: "Coextruded Films--Process and Properties" by John E. Guillotte.
Generally speaking, three methods of proceeding with the coextrusion of thermoplastic materials are known using conventional extruders whose number is equal to the number of polymers to be extruded. The first method consists of extruding the polymers separately and combining them as they exit the extrusion dies. The second method consists of feeding a single die by means of at least two extruders, with the die including as many passages as there are extruders and, consequently, polymers to be extruded. The streams of polymers meet at the level of the exit orifice of the die, thus perceptibly just prior to exiting from the latter. The third method consists of feeding a stream distributor by means of the desired number of extruders. In this distributor, the polymers combine into a single stream which is supplied to the die. In these processes, the respective flow rates of the extruders usually make it possible to control the relative thicknesses of the extruded polymers.
While numerous polymers can be thus coextruded, it has not been possible to satisfactorily combine PVF.sub.2 with other polymers by these techniques. The reason, it is believed, is probably due to the lack of compatibility of the PVF.sub.2 with the other polymers and to the well-known lack of adhesion between fluorine-containing resins and the majority of extrudable polymers, such as polyurethanes. In the past, the use of bonding agents has been required to adhere the PVF.sub.2 to the polymers.